This invention relates to the manufacture of semiconductor devices and, more particularly, to the manufacture of chrome masks for use in making semiconductor devices.
As is known in the art, semiconductor devices are formed by a variety of sequential steps in which a semiconductor wafer is subjected to various treatments such as doping, etching, oxidizing, or depositing various layers. Many, if not all, of these operations are not carried out on the entire wafer, only selected portions of the wafer. Which portions get treated depend upon the pattern formed in a layer of photoresist, typically an organic polymer, overlying the wafer.
The photoresist is patterned, as the name implies, by selectively exposing it to actinic radiation which causes polymerization of many of its molecules and, hence a change in etch rate between exposed and unexposed areas. The photoresist is then exposed to a suitable etchant to erode the photoresist and expose the underlying layer in the selected areas. The actinic radiation may comprise heat, visible light, ultraviolet light, X-rays or an electron beam. In some applications, such as using an electron beam, the wafer may be directly written upon or scanned by the electron beam to form the requisite pattern. More typically, a photomask is used. The photomask is usually a patterned chrome layer overlying a glass or quartz substrate. Radiation passes through the substrate and the shadow of the pattern on the photomask falls on the photoresist. The photomask may or may not contact the photoresist.
In the manufacture of semiconductor devices, particularly integrated circuits (IC's), the geometry of the patterns in the photomask is getting progressively smaller. The current state of the art is that the pattern is formed on the photomask by writing on the photomask with an electron beam. A problem has developed in that wet etch chemistries do not permit etching features smaller than about two microns. Wet chemical etches are isotropic. The result is significant undercutting of the photoresist, which changes the geometry of the pattern one is trying to form in the chrome layer. While tolerable for large geometry patterns, the undercutting becomes a significant percentage of the small geometry pattern. While a dry etch or plasma etch can provide an anisotropic etch, the electron beam (e-beam) photoresists used cannot tolerate a plasma. The photoresist, e.g. PBS (polybutenesulfone), decomposes in a plasma and definition is lost.
In view of the foregoing, it is therefore an object of the present invention to provide a process for etching chrome photomasks in a plasma reactor.
Another object of the present invention is to provide a process for etching high resolution chrome photomasks.
A further object of the present invention is to provide a plasma etch process for retaining definition despite decomposable photoresists.
Another object of the present invention is to provide a process for etching PBS in a plasma.
A further object of the present invention is to provide a process for producing a positive image from a positive photoresist